The advent of technology for storing images on digital media has increased the need for a method to protect against piracy. Images stored on prior forms of media (e.g. VHS, Beta, audiotapes, etc) are inherently degraded when copied. Images stored on digitally encoded media can be copied with no degradation; therefore, perfect copies of copies of copies, etc. can be made.
The introduction of Digital Versatile Discs (DVD) containing movies has created increased incentives for both casual and professional unauthorized copying. At the movie industry's urging, technology has been put in place to protect against simple duplication of DVD disks using equipment available to unsophisticated consumers. This is similar to the protection that exists which prevents one from duplicating a VCR tape by connecting together two commercially available VCRs.
While such protection mechanisms protect against some types of copying, a personal computer connected to a DVD device present a much more complicated problem. Open architecture devices such as personal computers reproduce the signals in the "clear" and such devices have many entry points, which can be used to duplicate material once it is in the "clear". The present invention uses digital watermarks to address the above described problem. The present invention also has other applications.
It is known that to facilitate the detection of digital watermarks one can insert a watermark signal that forms a grid. The grid can be used to determine orientation and scale. With the present invention the data signal and the grid signal are integrated into a single watermark signal in such a manner that the visual artifacts introduced by the watermark are minimized.
In applications such as DVD, an important factor that needs be considered is the bit rate of the bit stream. There are disadvantages if introduction of a watermark into a bit stream changes the bit rate. For example if images are going to be recorded on a medium such as a DVD disc, increasing the number of bits in the bit stream will decrease the number of images that can be recorded on a single disk. It is known that, in general, adding a watermark to a stream of images will increase the number of bits in the bit stream. The present invention provides a method and apparatus, which preserves the bit rate even though watermarks are introduced into the images.